reverseproxy: Allow using TLS for port 80 upstreams (see #3361)

An upstream like https://localhost:80 is still forbidden, but an addr of
localhost:80 can be used while explicitly enabling TLS as an override;
we just don't allow the implicit behavior to be ambiguous.

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -151,15 +151,6 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 			}
 		}
 
-		// if scheme is not set, we may be able to infer it from a known port
-		if scheme == "" {
-			if port == "80" {
-				scheme = "http"
-			} else if port == "443" {
-				scheme = "https"
-			}
-		}
-
 		// the underlying JSON does not yet support different
 		// transports (protocols or schemes) to each backend,
 		// so we remember the last one we see and compare them